Differential constant speed drives (CSD's) are used successfully in most military and commercial aircraft in use today, and their primary function is to maintain a constant speed input to a generator, when the engine varies over a 2:1 or 3:1 speed-range. While this approach, and the technology of variable-speed/constant-frequency (VSCF) have been viable for generators in the 20 to 120 kva range, the emergence of all electric airplane concepts may well dictate generator sizes in the order of 200 to 500 kva. As a consequence, the conventional constant speed drive and VSCF technologies will be adversely affected in weight, size, complexity, as well as thermal management.
Aside from the torque (and current sizing) problems that impact on weight and size of CSD's and VSCF systems, with large generators, there is the question of transmission-efficiency, which impacts on the quest for fuel efficiency. On the other hand, as has been described in co-pending U.S. patent application Ser. No. 173,111, filed July 28, 1980, for "A Direct-Driven Aircraft Generating System Providing Variable and Constant Levels of Electric Power", assigned to the assignee herein, the majority of the loads in a typical large aircraft can be powered by a generator system, whose voltage and power is proportional to frequency (engine speed).
Another consideration in design of aircraft power systems is that it would be highly desirable to utilize squirrel-cage rotor induction generators, which are extremely rugged and reliable machines. Such generators are inexpensive to construct, and they can be easily fabricated. There are, however, certain problems associated with induction generators. First, to operate as a generator, the rotor must be driven above its "synchronous speed"; i.e., in a negative-slip condition. Second, the machine cannot operate as a generator unless there is at least one synchronous power source in the system to excite it. Further, the machine cannot supply either its own excitation or any (lagging) reactive kilovars, reflected by the loads in the system.
A variety of differential and variable-speed drives, utilized in conjunction with electric machines, are known in the prior art. Such drives are typified by those disclosed in U.S. Pat. Nos. 2,810,844 to Morrill; 3,032,696 to Payne et al; 2,077,768 to Perry; 2,990,503 to Clark; and 2,153,252 to Hansdorf.
In the '844 patent, a constant speed drive type system is disclosed which uses a differential planetary gear arrangement to control the frequency of a dynamo/electric machine, which derives a selected frequency, different from the base frequency of the machine. The '696 patent discloses an engine driven generator/motor that includes a variable-speed drive interposed between the engine and generator. The drive system includes bi-directional (overrun) clutches to prevent direct drive of the transmission in the generator mode, and gear-reduction in the motor (start) mode. In addition, a current sensing control (current-transformer) is used to monitor and control the power taken by the motor/generator.
A synchronous drive system is disclosed in the '768 patent, wherein a frequency changer is driven at a variable speed, and rotary induction machines are electrically "locked in step" with the frequency generator. In this particular system, the machines operate at the speed of the frequency generator. The '503 patent, by contrast, describes a two directional power flow system that utilizes multiple devices such as a vari-drive, directional clutches, synchronous motor/generator, etc. In this system, the synchronous machine is automatically controlled when in a driving mode of operation, such that the ratio change in the drive gears is adjusted in dependence upon the torque and power of the driving motor, as ascertained by the magnitude of the current drawn by the machine. The system operates to maintain the magnitude of the current at a predetermined level.
Finally, a vari-drive (toroidal-type) transmission is shown in the '252 patent, driving an alternating current machine. The transmission allows for the supplying of power at various selective amounts from a constant speed
Another co-pending U.S. patent application Ser. No. 220,865, for "Variable-Speed Drive For Frequency Control Of Induction Generators," assigned to the assignee herein, utilizes a synchronous generator, as a separate (integral) exciter for an induction generator is disclosed. The system includes the capability of adjusting the slip-frequency as a function of the load over the speed range, but it does not operate in a constant-speed mode.
Yet another co-pending U.S. patent application, Ser. No. 234,761, for "All Electric Constant Speed/Variable Speed Drive/Generator Assembly," assigned to the assignee herein, discloses a relatively lightweight and compact drive/generator assembly which includes a "make-up" induction machine and a permanent magnet generator. This drive-generator assembly, differs from the drive-generator assemblies, described in this instant invention, in that it is "all electric" and does not use toroidal drive, or other mechanical elements in the mechanical power train to the planetary gearbox. However, the mechanical (toroidal type) elements are in a more advanced stage of development and have in fact a heritage of extensive use in industrial applications. To this extent, the instant invention enjoys practical advantage over the co-pending patent.
It is a primary object then, of the present invention, to provide a drive system for large generators as may be used in aircraft power generation systems; such a drive system will minimize the adverse effects of weight, size, complexity, and thermal management, normally associated with CSD and VSCF systems.
It is another object of the present invention to provide an aircraft power generation system in which the motor, and provides a means for varying the proportionate actual speeds of rotating field and armature members of an electric machine.
All of the aforementioned prior art patents are directed to power systems quite different from the dual-range variable-speed drive and variable-speed differential drive systems for frequency control of generators, as described by the present invention.
A somewhat simple system which utilizes a synchronous generator as an exciter for an induction generator is disclosed in co-pending U.S. patent application Ser. No. 220,371, for "Induction Generator/Dual-Samarium-Cobalt Generator Combination," assigned to the assignee herein. The exciter generator and induction machine disclosed therein are driven through a fixed ratio gear system which causes the exciter generator to run at a slower speed relative to the induction machine. Under this arrangement, the induction machine operates as if it is in an overspeed condition and thereby functions as a generator. While this relatively non-complex system is quite desirable for certain uses, it is somewhat limited in that it is insensitive to the different loads that may be imposed on the induction generator. By contrast, the present power system, while utilizing a highly desirable induction generator as a primary source of power, also provides the added feature of being able to adjust the negative slip-frequency. This flexibility is particularly desirable when the power system is utilized in aircraft, where loads may vary over a wide range. The ability to control the slip-frequency thereby adapts the induction-generator to meet the variable demands of the loads. drive-generator operates in a dual-range over the speed range of the aircraft's engines.
It is a further object of the present invention, to provide a differential variable-speed drive mechanism, of reduced weight and complexity, that will control the excitation-frequency of an induction-machine, when it operates in a dual-range generating mode.
It is a further object of the present invention, to provide a multi-generator (aircraft) power source that includes an induction-generator for providing dual-range primary power, an exciter machine, and a differential variable-speed drive, that controls the negative slip.
These and other objects of the present invention will become more readily apparent after a full consideration of the following description of the instant invention, and the several advantages attendant thereon.
The disclosures of all U.S. patents and co-pending patent applications referenced herein are hereby incorporated by reference.